1 . Field of the Invention
The present invention relates to an operation mechanism of a variable displacement hydraulic pump in a hydrostatic transmission (hereinafter referred to as an “HST”) adapted as a shift transmission for a vehicle, wherein the operation mechanism is attempted to reduce a shock in shifting the vehicle effectively, to have a good reliability in its motion and to ease assembly and maintenance.
2. Background Art
As well-known, there is a conventional HST comprising a hydraulic pump and a hydraulic motor fluidly connected with each other, wherein at least the hydraulic pump is volumetrically variable so that the capacity of the hydraulic pump is varied for changing the traveling speed of a vehicle. The hydraulic pump is provided with a capacity regulating member, e.g., a movable swash plate, operatively connected with a speed control device like a lever or a pedal on a vehicle. The capacity regulating member is operated in correspondence to the operational degree of the speed control device so as to changing the traveling speed of the vehicle.
Furthermore, conventionally, there are various well-known means to moderate the operation of the capacity regulating member for avoiding sudden shock in shifting as follows:
Japanese Utility Model Laid Open Gazette No. Hei 3-69,755 discloses a damper provided in an HST housing, wherein a rod of the damper is pivotally connected to a speed control arm for operating the capacity regulating member. Lubrication oil filled in the housing is introduced into the damper. In the damper, the flow of lubrication oil is limited by an orifice or the like. Thus, the damper serves as a fluid-resistive device which uses lubrication oil.
Japanese Utility Model No. Hei 7-16,138 discloses a gas damper replacing the above-mentioned damper, wherein the gas damper using air or the like is disposed in the HST housing, however, out of communication with lubrication oil in the housing.
Japanese Utility Model No. Hei 6-12,318 discloses a damper serving as a fluid-resistive device using HST-operation oil.
However, since the fluid-resistance generated by the damper which uses lubrication oil or HST-operation oil in an HST housing is small, the moderation of the capacity regulating member in motion may be insufficient.
The gas damper is also desired to be improved because of its small resistance and its large elasticity which causes uncertainty in its damping effect.
Furthermore, the above-mentioned conventional dampers cannot be assembled easily, thereby increasing the number of processes and labour for producing the HST.